In TDMA radio communication systems (TDMA=Time Division Multiple Access) information is transmitted on a channel in the form of signal frames, that are transmitted by the transmitter during evenly distributed time intervals. In the spaces between these signal frames the transmitter is "silent". In order to synchronize the receiver to these signal frames each signal frame comprises a known synchronization word in predetermined positions within the signal frame. In, for example, the European GSM system for mobile telephony this synchronization word is 26 bits long. When the receiver expects a new signal frame from the transmitter a training sequence, that is identical to the 16 central bits of the synchronization word, is generated by a training sequence generator in the receiver. The received signals are compared to the locally generated training sequence, and when the best possible correlation is obtained between this sequence and the received signals synchronization is considered to exist between the locally generated and the received signal.
In addition to the synchronization itself the training sequence is also used for channel estimation. Since the radio channel often is subjected to multiple path propagation the receiver comprises some sort of equalizer to eliminate this phenomenon. The equalizer requires a time limited estimate of the impulse response of the channel. This impulse response can be obtained from the correlation signal. For this purpose not only a synchronization position but an interval or a window defining the multiple path propagation and indicating where the equalizer is to operate is required.
A previously known method of finding the position of the window of the equalizer is to choose that interval of fixed length that contains most of the energy from the correlation. A drawback of this method is that disturbances in the received signal can give a correlation peak far away from the actual peak. For this reason the disturbance has a large instantaneous influence on the position of the window and creates a variance (uncertainty) in this position, which results in deteriorated receiver performance.